The present invention relates to the measurement of electrical signals and, in particular, the measurement of characteristics of modulated orthogonal signals.
A common method of transmitting digital information is to modulate two orthogonal signals with digital information. The transmitted signal may then be considered to be composed of a real component and an imaginary component. Quadrature amplitude modulation (QAM) is an example of such a technique, where two sine waves, ninety degrees out of phase (e.g., sine and cosine) are modulated by the digital information.
A useful tool for examining the performance characteristics of a digitally modulated signal formed of a first and a second orthogonal component is a constellation diagram. The signal of interest is plotted on two axes, one for the real (e.g., in-phase) component and one for the imaginary (quadrature-phase) component. For an error-free signal (e.g., no noise, no gain errors, no symbol clock error, no frequency offsets), the constellation diagram will simply be a pattern of points (e.g., 64-QAM—rectangular grid, 8-PSK—circle), each point representing a possible digital value. In practice, the points are typically blurred by errors into clusters. The first and second orthogonal components may be provided by demodulating the modulated signal with respect to a respective orthogonal signal. The constellation diagram can then be displayed on a suitable display device, such as an oscilloscope. A specialized instrument that demodulates the signal and displays the diagram may also be employed.
Referring to FIGS. 1 and 2, examples of a constellation diagram for two 64-QAM signals are shown. The signal of FIG. 1 has very little error and thus the clusters appear nearly as points. The signal of FIG. 2 has a large error content resulting in large clusters.
The usefulness of constellation diagrams often lies in the size and shape of the clusters. The size of the clusters around each constellation point gives a direct estimate of the Error Vector Magnitude (EVM) and the shape of the clusters can indicate problems such as gain errors between the in-phase and quadrature-phase processing, frequency offset and symbol clock error. The clusters thus provide an error measure for the modulated signal. However, the example clusters of FIG. 1 are so small that the shapes are difficult to assess. In contrast, the clusters of FIG. 2 are so large that overlap occurs. In both cases, it is difficult to assess the individual cluster shapes.
It is known to provide a zoom function for a constellation diagram displayed on a specialized measurement instrument to allow portions of the diagram to be enlarged for better viewing. However, when zoomed in on a cluster, the ability to assess the overall diagram and most other clusters is lost.